1. Field of the Invention
This invention generally relates to a vehicular optical radar apparatus adapted to receive light, which is reflected by and returned from an object (or body) when scanning the object in a horizontal direction with the light emitted in a direction from a vehicle, and to compute the distance from the vehicle to the object on the basis of the time difference between the emission of the light and the reception thereof and to detect the direction of the object from a scanning angle of the emitted light when the reflected light is obtained, and, more particularly, to a vehicular optical radar apparatus adapted to enhance the actual operating efficiency in performing a distance computing operation.
2. Description of the Related Art
Hitherto, there has been well known a vehicular optical radar apparatus for detecting the distance between a vehicle and an object and the direction of the object on the basis of light reflected from the object which is scanned with a luminous flux emitted from the vehicle. Such a conventional vehicular optical radar apparatus is widely used in vehicle-mounted systems such as a vehicle-surroundings monitoring system and an inter-vehicle distance control system.
Conventional vehicular optical radar apparatus of such a type is adapted in such a manner as to detect a target object in a wide range by reducing blind spots as a result of scanning the target object in a horizontal direction with emitted light by the use of a rotary mirror oscillating by a predetermined angle, as illustrated in, for example, the Japanese Unexamined Patent Publication No. 5-113481.
Generally, a device provided with a mirror and adapted to rotate the mirror within a range of a predetermined angle to thereby oscillate the mirror is used as scanning means of the conventional vehicular optical radar apparatus. Further, such conventional scanning means is adapted to simultaneously scan both of a light transmitting optical system and a light receiving optical system, because of the fact that no devices contrived in such a way as to have a wide-angle field of view have been proposed as the light receiving optical system.
However, in the case of using the scanning means adapted to oscillate the mirror, the scanning means performs a reciprocating angular motion. This results in the presence of a dead center (position) (or dead point) at which the direction of the motion is changed. Thus, the stability of the angular motion is largely degraded. Namely, the bounding (or saltation) of the scanning means at the dead center causes an impact force to thereby generate an oscillation thereof. Consequently, the angular velocity becomes unstable.
Moreover, in the case where the forward movement and the backward movement of the reciprocating angular motion are different in the angular velocity from each other, if it is assumed that the angular velocity in the forward movement is lower than the angular velocity in the backward movement during a distance measuring operation performed in such a case, a quick return (or reverse) operation is performed at a high angular velocity in the backward movement because no measurement is performed during the backward movement.
However, the presence of quick return time periods in a sequence of distance measuring operations indicates that the quick return time periods are dead (or wasteful) time during which the distance cannot be measured. Thus, in the case where the length of time of one cycle (or period) is limited, a measuring time is reduced, so that the angular velocity is increased. This directly leads to deterioration in lateral or transverse resolution of an azimuth angle. Consequently, the conventional vehicular optical radar apparatus has such a fatal defect in the radar performance thereof.
Furthermore, in the case that a distance measuring operation (or distance measurement) is performed during each of the forward and backward movements of the reciprocating motion of the scanning means, if there is caused an error in angular position between the forward and backward movements of the angular motion, even when the position of a stationary target object is detected, the detected positions thereof respectively corresponding to the forward and backward movements differ from each other. Moreover, the position of the object detected in this manner changes repeatedly every cycle of the distance measuring operation. Thus, the performance of the conventional vehicular optical radar apparatus is clearly presented. Consequently, the position of a target object cannot be detected with a high degree of precision.
Meanwhile, in the light receiving optical system of the vehicular optical radar apparatus, an angle of view (or field) for receiving light is generally determined by the focal length of the optical system and the dimensions of a light receiving element (or device). In the case of using a sensitive avalanche photodiode (hereunder abbreviated as "APD") as a light receiving element, owing to the physical characteristics of APD element, it is difficult to manufacture relatively-large-size APD elements. It is, therefore, impossible to obtain a desired angle of view.
Further, in the case of the vehicular optical radar apparatus adapted to simultaneously scan the light transmitting optical system and the light receiving optical system as above described, it is possible to detect a desired scanning range. However, the configuration of the entire apparatus is considerably complex.
For instance, to take a cam-link mechanism, in which a light transmitting mirror and a light receiving optical system are configured through a rigid link and this link is caused by a cam to perform a reciprocating angular motion, as an example, there is the need for providing not only an axle, which constitutes the fulcrum of the link, a bearing and a washer but a spring and a damper, which are used for suppression of vibrations to be caused just after reaching the dead center of the reciprocating angular motion, in the vehicular optical radar apparatus.
Moreover, in the case of using a scanning means of the omnidirectional scan type (namely, the rotary type), when a desired scanning angle is relatively small, the dead time, during which no measurement is performed, is overwhelmingly longer than the actual measuring time in one cycle (corresponding to 360.degree.) Consequently, the actual operating efficiency is extremely degraded.
As above described, the conventional vehicular optical radar apparatus uses the scanning means of the reciprocating angular motion type, so that there is the dead center due to the reciprocating angular motion. The conventional vehicular optical radar apparatus, thus, has encountered a problem in that the stability of the angular motion is largely degraded.
Further, the conventional vehicular optical radar apparatus has encountered another problem in that if a time period, during which the backward movement of the reciprocating angular motion of the scanning means is performed, includes dead time during which the distance cannot be measured, and if the length of time of one cycle is limited, a measuring time is reduced, and as a result, deterioration in lateral or transverse resolution of an azimuth angle is caused.
Moreover, the conventional vehicular optical radar apparatus has encountered still another problem in that if the distance measuring operation is performed during each of the forward and backward movements of the reciprocating motion of the scanning means, and if there is caused an error in angular position between the forward and backward movements of the angular motion, the detected positions of a target object, which respectively correspond to the forward and backward movements come to differ from each other, and thus, the position of a target object cannot be detected with a high degree of accuracy.
Furthermore, the conventional vehicular optical radar apparatus has encountered yet another problem in that when using a sensitive APD as a light receiving element, it is difficult to manufacture relatively-large-size APD elements, and therefore, it is impossible to obtain a desired angle of view.
Additionally, the conventional vehicular optical radar apparatus has encountered a further problem in that when enabling the detection of a desired range by simultaneously scanning the light transmitting optical system and the light receiving optical system, it is necessary to use a cam-link mechanism having a large number of parts, and thus the configuration of the entire apparatus is complex.
Besides, in the case of using a scanning means of the omnidirectional scan type (namely, the rotary type), when a desired scanning angle is relatively small, the dead time, during which no measurement is performed, is overwhelmingly longer than the actual measuring time included in one cycle (corresponding to 360.degree.). Consequently, the conventional vehicular optical radar apparatus has encountered still another problem in that the actual operating efficiency is extremely degraded.
The present invention is accomplished to solve the aforementioned problems of the conventional vehicular optical radar apparatus.